1. Field of the Invention:
The present invention relates to a power transmission device and more particularly, it relates to a power transmission device capable of transmitting drive force through the belt, chain or the like, absorbing the eccentric movement and the like of the pulley.
2. Prior Art:
In the case of the grinder and machine tools for centerless machining, and machines for conveying goods, for example, power transmission to rotate rollers is usually achieved by the belt drive system. FIGS. 1a and 1b show an example of this power transmission, citing the jackshaft portion of the centerless grinder. The power transmission from a drive motor 1 to a jackshaft 2 is carried out by pulleys 5 and 6 attached to an output shaft 3 of the drive motor 1 and a roller shaft 4 of the jackshaft 2, respectively, and belt members such as V-belts 7 stretched between the pulleys 5 and 6. The jackshaft 2 includes a U-shaped frame 8, a roller 9 rotatably supported by the frame 8 through a roller shaft 4, and a case filled with oil to smoothly rotate the roller shaft 4 supported by the frame 8.
FIG. 2 shows in detail how the pulley 6 is attached to the roller shaft 4 in the case of the conventional jackshaft 2. The roller shaft 4 is tapered, that is, it is made larger and larger in diameter as it goes to its base, and it is provided with a thread portion 4b at the foremost end of its tapered portion 4a. It is also provided with a shoulder portion 4c, larger in diameter, at the base of its tapered portion 4a and it further extends toward the roller 9 (or left in FIG. 2). The pulley 6 has a tapered opening fitting to the tapered portion 4a of the roller shaft 4 and it is also provided with a recess 14 at its foremost end which houses a fastening member for attaching it to the roller shaft 4. Key grooves 12 each extending in the longitudinal direction are provided on the outer circumference of the tapered portion 4a of the roller shaft 4 and also on the inner circumference of the tapered opening of the pulley 6. The pulley 6 is fitted onto the roller shaft 4, a key 13 is put into the key grooves 12 and a nut 11 which serves as the fastening member is screwed onto the thread portion 4b of the roller shaft 4. The pulley 6 is thus fastened and fixed integral to the roller shaft 4 in such a way that its base end face is not struck against the shoulder portion 4c of the roller shaft 4 but positioned adjacent to it. When the V-belts 7 are stretched between the pulley 6 thus fixed and the pulley 5 positioned on the side of the drive motor, the power transmission system to the jackshaft 2 is completed accordingly.
In the case of this conventional power transmission system, however, attaching of the pulley 6 to the driven roller shaft 4 is achieved by closely fitting the tapered opening 6a of the pulley 6 onto the tapered portion 4a of the roller shaft 4. Therefore, some influence which is caused by or given to the pulley 6 at the time of power transmission is added to the roller shaft 4 to badly affect the movement of the jackshaft 2. The roller shaft 4 and the pulley 6 are rotating members in the jackshaft 2. It is therefore the best that their rotating center axes are consistent with each other as shown by 0 in FIG. 2. In practice, however, their rotating center axes are shifted from each other because of working error, impact added to them after the completion of the device, and the like, thereby causing eccentricity as shown by a two-dot and dash line 01 in FIG. 2. Even when their rotating center axes are consistent with each other, that face of the pulley 6 with which the belt is contacted may not be made as a true circle. The possibility is caused in this case that unreasonable force is added to the pulley 6 to force the roller shaft 4 to one side. The device is thus gradually made loose.
When the conventional couplings are used as a transmission manner, it is almost impossible that the driving and driven couplings are on a same line. Even if the clearance is made larger between them, the possibility is high that the roller shaft is forced to one side. Further, when minus torque acts, the V-belts 7 are irregularly rotated because the clearance is large.